The present invention relates to a technology effective for application to a power supply circuit for a high frequency power amplifier, which is used in a wireless communication apparatus such as a cellular phone or the like, and particularly to a power supply circuit available for a cellular phone having a plurality of systems such as a GSM (Global System for Mobile Communication) system, a CDMA (Code Division Multiple Access) system, etc.
As a power supply circuit for a high frequency power amplifier of a cellular phone, there has currently been known a technology wherein a voltage converter circuit like a DCxe2x80x94DC converter and a power switch comprising MOSFET or the like are utilized in combination, and when output power is low, the power switch is turned off and a voltage stepped down by the DCxe2x80x94DC converter is used, whereas when the output power is high, the DCxe2x80x94DC converter is turned off and the power switch is turned on to supply a battery voltage to the high frequency power amplifier as a source or power supply voltage as it is, thereby enhancing power efficiency of the whole system.
On the other hand, since the GSM system is higher in output power than CDMA, a power supply circuit using a DCxe2x80x94DC converter is not provided, and an APC (Automatic Power Control) circuit is provided which effects feedback onto a gate bias circuit which detects a DC level of an output and generates a gate bias voltage of an output power element so as to reach output power necessary for calling (see, for example, Unexamined Patent Publication No. 2000-151310). Such a control system is generally called xe2x80x9cclosed loop system or typexe2x80x9d.
However, the system of controlling the output power according to the closed loop type has the problem that since there is a need to provide the APC circuit, a circuit scale increases correspondingly and a packaging density is reduced. Therefore, there is provided a system for controlling a source or power supply voltage of an output power FET based on a signal for designating an output level, so that the output level changes in proportion to the signal to thereby linearly operate the output power FET, thus ensuring linearity of an output of a high frequency power amplifier. This system is called an xe2x80x9copen loop system or typexe2x80x9d and has the advantage of enabling a reduction in circuit scale as compared with the closed loop system.
A standard DCxe2x80x94DC converter incapable of performing complex control is now used in a CDMA-system cellular phone. However, if a power supply circuit is configured using the standard DCxe2x80x94DC converter even though the cellular phone is considered to proceed toward more multi-functioning from now on, it is difficult to cope with the multi-functioning in this way. If an attempt is made to configure a power supply circuit capable of performing the complex control by using standard electronic parts, then the number of parts increases and a packaging density is reduced, thus causing difficulty in reducing the size of the cellular phone. Although the cellular phone is moving toward considerably achieving a reduction in power consumption, there has also been a strong need for a further reduction in power consumption. The reduction in power consumption has heretofore been made with an eye to enhancing the efficiency of a high frequency power amplifier. However, only the enhancement of the efficiency of the high frequency power amplifier makes unattainable a sufficient reduction in power consumption.
An object of the present invention is to provide a power supply circuit for a high frequency power amplifier, which is capable of reducing power consumption and thereby increasing a call time of a cellular phone and the life of a battery.
Another object of the present invention is to provide a power supply circuit for a high frequency power amplifier, which is capable of being used in a cellular phone capable of communicating according to a plurality of systems like a GSM system and a CDMA system and providing a reduction in power consumption.
A further object of the present invention is to provide a power supply circuit for a high frequency power amplifier, which is capable of reducing the number of parts and thereby reducing its size in a case where the power supply circuit capable of performing complex control with multi-functioning of a cellular phone is configured.
The above, other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings.
Summaries of typical ones of the inventions disclosed in the present application will be described in brief as follows:
A power supply circuit, which supplies a power supply voltage to a high frequency power amplifier, comprises a voltage converter circuit of a switching power type like a DCxe2x80x94DC converter, a voltage control transistor like an FET, and a power control circuit which controls the voltage control transistor. When the power supply circuit is used in a CDMA-system cellular phone, it supplies a power supply voltage generated by the DCxe2x80x94DC converter to the high frequency power amplifier upon a low output, whereas upon a high output, the power supply circuit turns on the voltage control transistor to directly supply a battery voltage to the high frequency power amplifier. On the other hand, when the power supply circuit is used in a GSM-system cellular phone, the power supply circuit supplies a power supply voltage generated by the DCxe2x80x94DC converter to the high frequency power amplifier upon a low output. Upon a high output, the voltage control transistor is controlled by a control circuit based on a signal for designating an output level to thereby generate a power supply voltage supplied to the high frequency power amplifier.
According to the above means, the power supply circuit for the high frequency power amplifier can be shared between the CDMA-system cellular phone and the GSM-system cellular phone, for example. Thus, the number of parts constituting the power supply circuit of the cellular phone configured so as to be capable of performing communications of at least two systems can be reduced, and hence the cellular phone can be brought into less size. Since the efficiency at the low output is enhanced in GSM-system communications, current consumption decreases and the cellular phone using the power supply circuit is capable of increasing a call time and the life of a battery.
Further, a power supply circuit for a high frequency power amplifier, which is used in a CDMA-system cellular phone configured so as to be capable of detecting an output level of the high frequency power amplifier to thereby control a bias voltage of the high frequency power amplifier, is made up of a DCxe2x80x94DC converter, a voltage control transistor, and a control circuit for controlling the voltage control transistor. In a first mode or a system (closed loop) in which the output level of the high frequency power amplifier is detected to control the bias voltage of the high frequency power amplifier, a power supply voltage produced by the DCxe2x80x94DC converter is supplied to the high frequency power amplifier upon a low output, whereas upon a high output, the voltage control transistor is turned on to directly supply a battery voltage to the high frequency power amplifier. On the other hand, in a second mode or a system (open loop) for controlling the power supply voltage for the high frequency power amplifier without depending on a signal for detecting the output level of the high frequency power amplifier, the voltage control transistor is controlled based on a signal for designating or specifying the output level to thereby generate a power supply voltage corresponding to the output level designating signal, followed by supply to the high frequency power amplifier.
According to the above means, the power supply circuit for the high frequency power amplifier can be shared between, for example, a mode or system in which a GSM-system cellular phone performs transmission in a closed-loop system or type, and a mode or system in which it performs transmission in an open loop system or type. Thus, the number of parts constituting the power supply circuit of the cellular phone configured so as to be capable of performing communications of at least two systems can be reduced, and hence the cellular phone can be brought into less size.
Preferably, a filter circuit using a ferrite bead as an inductor is provided at an output terminal of the DCxe2x80x94DC converter. Thus, noise produced due to a switching operation of the DCxe2x80x94DC converter can be cut.
Further, preferably, the power control circuit for controlling the voltage control transistor based on the output level designating signal makes use of a circuit which comprises an amplifier (operational amplifier circuit) that outputs a voltage corresponding to the output level designating signal, and a MOSFET having a gate controlled based on the output of the amplifier to output a source or power supply voltage of an output power FET from its drain, and which feeds back the drain voltage of the MOSFET to the amplifier to thereby generate a desired power supply voltage. When the voltage control transistor is on-off controlled according to the closed-loop system, the output of the amplifier is brought to full amplitude to thereby perform its on-off control. Thus, the response to a control signal is improved.
Further, when the DCxe2x80x94DC converter makes use of a synchronous rectifying circuit configured so as to complementarily on-off control a first switch element and a second switch element connected in series between a voltage input terminal and a reference potential terminal to allow a current to flow through an inductor connected between a connecting node of the first switch element and the second switch element and an output terminal, thereby outputting a voltage obtained by stepping down the voltage applied to the voltage input terminal, the second switch element is controlled so as to avoid its turning on when the output level designating signal indicates a predetermined level or less, thereby outputting a desired voltage under a switching operation of the first switch element. Thus, the output of the DCxe2x80x94DC converter varies between a power supply voltage (Vdd) and a negative potential (xe2x88x920.7V) and hence an operation margin for the DCxe2x80x94DC converter can be enhanced.